1. Field of the Invention
The present invention relates to a process of producing nucleic acid derivatives which are useful as an antiviral agent. More specifically, the present invention relates to a method for purifying nucleic acid derivatives obtained by alkylating a purine base, and intermediates in the purification thereof.
2. Description of the Background
(-)-9-[1'S,2'R-Bis(hydroxymethyl)cyclopropan-1'-yl]methylguanine represented by formula (II) ##STR2## is known to have a high antiviral activity (Japanese Patent Laid-Open No. 78,357/1993).
The above-mentioned compound is produced by a process in which a protected purine base is alkylated as described in Japanese Patent Laid-Open Nos. 80,670/1994 and 316,155/1995. This alkylation occurs in both the 7- and 9-positions of the purine ring. In order to obtain a desired 9-substituted compound of the present invention, the 7-substituted compound and the unreacted purine base have had to be separated chromatographically.